An introduction to Petrophysics

The term “petrophysics” was coined over 60 years ago by G.E. Archie (of the famous Archie Equation, more of which anon.) and J.H.M.A. Thomeer in a quiet bistro in The Hague. By their definition, petrophysics is the study of the physical and chemical properties of rocks and their contained fluids.

Petrophysics emphasizes those properties relating to pore systems, their fluid distribution and flow characteristics. These properties and their relationships are used to identify and evaluate:

Hydrocarbon reservoirs

Hydrocarbon sources

Seals

Aquifers

To put it another way: The role of the petrophysicist is to establish the quantity of moveable hydrocarbon in the near-well bore region in both new and historical wells, and to communicate his/her findings to the geologists and engineers. The team then finds ways to exploit those hydrocarbons turning them into money.

The petrophysicist is therefore a team member – usually of more than one team: a team of explorationists, a field development team, and maybe a reservoir management team. The petrophysicist provides information needed by team members, as well as physical and chemical insights required by other team colleagues.

Understanding what these characteristics mean, and their significance in the assessment of reserves is not too hard. The difficult part is quantifying the above properties at a level of certainty needed to make economic decisions leading to development and production. The seven characteristics listed are interdependent: the science of petrophysics unscrambles this interdependency in the subsurface.

I have mentioned ‘near wellbore region’ and will mention ‘wireline log’:

a log (whether it be wireline or from LWD (logging-while-drilling)) is a depth-indexed survey of a particular physical or chemical property of the rocks the drillers have succeeded in penetrating. By depth-indexed, I mean the values of that property are provided (usually) every six inches, 15.24cms. The properties measured are all useful in determining the reservoir characteristics which need to be known before any economic decisions can be made. For example, the velocity of sound: the slower the velocity, the more porous (in general) the rock formation. Another example is the level of natural radioactivity exhibited by the rock: the higher the gamma ray count, the higher the shale content (although that is by no means always the case), and the less prospective, or attractive that formation is.

near wellbore region: the logs referred to above are obtained from oilfield tools lowered into the well. These services are provided by a number of well-known contractors: Schlumberger, Baker Hughes, Halliburton, and Weatherford. They acquire the logs of the reservoir properties (at a considerable cost) which the petrophysicist then interprets in terms of reservoir property information for his team colleagues. Since the petrophysicist is responsible for the logs, it is he who usually witnesses/supervises their acquisition. This is one of the more interesting aspects of the petro’s life, sometimes involving travel to the wellsite in some distant corner of the planet, but that is now less common, thanks to the Internet and remote witnessing.

The ‘near wellbore region’ qualifier comes about because the logging tools only sense a short distance away from the wellbore. The precise distance varies from tool to tool, but can be less than ca.8-9 cms for the current magnetic resonance scanners to greater than 1m for resistivity tools.

So, armed with a suite of suitable, good quality logs, the petrophysicist is in a good position to address most of the properties listed above. However, no tool has been invented which can measure permeability on a continuous depth-indexed basis. We can, however, make educated estimates using the logs and other sources of information.

Expanding upon data sources for the petrophysicist:

Mud logging solids (drill cuttings), liquids entrained in the mud apart from the mud itself, and gas content of the mud

Petrophysics emphasizes the integration of core data with log data; the adjustment of core data to reservoir conditions; and the calibration of core data to log data. The goal of the calculations is to use all available data, calibrated to the best standard, to reach accurate quantitative values of the required petrophysical parameters.

In practical terms, petrophysics is used for: determination of original hydrocarbons in place [original oil in place (OOIP) or original gas in place (OGIP)] and their distribution, and reservoir engineering dynamic flow calculations. For the development geoscientists, petrophysics means quantifying the detailed stratigraphic, depositional, pore fabric and fluid descriptions of the reservoir. To make calculations of OOIP or OGIP and the various flow calculations, accurate foot-by-foot calculations of lithology, net pay, porosity, water saturation, and permeability are necessary. The above is summarised in the equation below (no petrophysical discourse is complete without at least one equation):

N/G : Net-to-gross – defines net sand, net pay

Sw: water saturation. 1-Sw is therefore the hydrocarbon content

Φ: Porosity – the percentage volume of the reservoir which contains fluids

Those three properties are all defined by the petrophysicist.

GRV (gross rock volume) is the subject of geological and geophysical deliberations.

Dr. Nick Colley is an expert in the Petrophysical interpretation of clastics and carbonates, and the integration of Petrophysical core analysis data. He has worked on reservoirs around the world from Trinidad to Australia via the UK, Norway, India, Libya, Nigeria, Palestine, Brazil, Bolivia, Kazakhstan and the East Indies. He worked for BG Group for more than 30 years and is now supporting OPC to develop the petrophysics service.

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